Mississippi Science P.7.5E.3

Science7th GradePhysical Science

The Standard

P.7.5E: Students will demonstrate an understanding of the law of conservation of mass.

Compare and contrast balanced and unbalanced chemical equations to demonstrate the number of atoms does not change in the reaction.

2026 Mississippi College- and Career-Readiness Standards for Science · Organization of Matter and Chemical Interactions

Teacher's field guide

What Students Need to Do: Students count each kind of atom on both sides of an equation. They identify which equations are balanced. They explain that an unbalanced equation fails to show atom conservation, not that atoms vanish.
What Mastery Looks Like: Given 2H2 + O2 → 2H2O, a student counts four hydrogen atoms and two oxygen atoms on each side. They can fix H2 + O2 → H2O by adding coefficients without changing subscripts.
Common Misconceptions: Students may change H2O to H2O2 to make the counts match, but that changes the substance. Others count molecules instead of atoms or think an unbalanced equation means atoms disappear.
How to Assess It: Exit ticket: Compare H2 + Cl2 → HCl with H2 + Cl2 → 2HCl. Count each kind of atom on both sides, then explain which equation is balanced.

Lesson moves

Give pairs colored counters to model 2H2 + O2 → 2H2O, then have them count each kind of atom on both sides.
Ask students to write why changing H2O to H2O2 makes H2 + O2 → H2O balanced but changes the product.
Have pairs sort cards showing H2 + Cl2 → 2HCl and H2 + O2 → H2O, writing atom counts beside each.
Show a rusty nail, then have students compare Fe + O2 → Fe2O3 with 4Fe + 3O2 → 2Fe2O3 by counting atoms.

Free download

A ready-to-print activity worksheet aligned to P.7.5E.3, with an answer key for the teacher on its own page. No account needed.

PDF, US Letter, prints cleanly in black and white.

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